for my planned hybrid, i will need to seal threads up to 3" and my PTFE tape says its only suitable for threads up to 2", i'm guessing because you would need miles of it to seal large threads, so what could i use?

I have used polyurithane roofing sealent to seal threads on a 2" pipe. It Works ,I have never had a leak. Im sure it can stand some kind of heat because roofs get kinda hot in the summer. It also stays flexiable,and sticks to almost anything.